Is the rhizome covered with substrate? If so, it shouldn't be! Only bury the roots.

If this is not the case, I suspect it is an iron deficiency. Are you using any kind of fertilizer? If not, a general fertilizer such as Tetra Florapride (I think that's the name...) should do the trick.

If you are already using fertilizer, check out Flourish Iron. Follow directions and you should be good. You will need an iron test kit if you go this way.